1. Field of the Invention
This invention relates to a TiO.sub.2 --SiO.sub.2 film (a film of titanium dioxide containing silicon dioxide) giving a film of anatase by heat treatment, and a method of forming the same.
2. Description of the Prior Art
Titanium dioxide (TiO.sub.2) is known as a substance which can be used repeatedly as a catalyst having a high activity in environmental cleaning, for example, for decomposing the exhaust gases of automobiles and the nicotine of tobacco.
TiO.sub.2 usually occurs in crystal form, and is classified by its structure into three types: rutile, brookite and anatase. It is easy to obtain rutile or brookite type TiO.sub.2 by heat treating a film of amorphous TiO.sub.2 at a temperature of 816.degree. C. to 1040.degree. C., but neither rutile nor brookite type TiO.sub.2 has a very high catalytic power. Anatase type TiO.sub.2 has a high catalytic activity, but is expensive, since it is mainly prepared by the hydrolysis of an organic titanium compound at a low temperature.
When TiO.sub.2 is used as a catalyst, it is necessary to form it into a film, or fine particles to provide a large surface for the donation and acceptance of electrons, and it is necessary to heat treat such a film to ensure its adhesion to a substrate. Anatase type TiO.sub.2 obtained by hydrolysis at a low temperature, however, is stable only at a temperature lower than 816.degree. C., and easily changes to rutile type if heated at or above 816.degree. C.